A blood brain barrier was postulated by Goldmann in 1909 to explain his observation that intravenously injected trypan blue stained all tissues except the central nervous system (CNS). As is well known, this concept has been the basis of much subsequent work in pharmacology and physiology. Less familiar is its use by virologists concerned with the pathogenesis of CNS infections.

Many viruses, when inoculated directly into the brain of a laboratory animal, are capable of producing encephalitis. A number of such viruses, however, cause no CNS disease when given intraperitoneally or intravenously, even when extraneural replication results in marked viremia. This fact alone could suggest that the postulated barrier prevents viral access to and multiplication in the brain. In addition, at least two other experimental findings fit the concept equally well: (1) Experimental injury, either direct or peripheral, results in changes which allow staining by vital dyes of localized areas